In the wake of Fremantle's 11-point loss to Sydney on Sunday, King declared Longmuir must guide the Dockers to finals action this year to show he is "the man" to lead the club forward.
"Is he the guy to take them all the way to a premiership? Sixth year, I have my doubts. I've had my doubts for a while," King told Fox Footy during his extended critique of Longmuir.
The Dockers had won six games in a row before falling to Sydney.
Although that defeat dropped Fremantle to ninth on the ladder, their 10-6 record means they are just a win and percentage adrift of the top four ahead of Saturday night's crunch home clash with Hawthorn.
Longmuir is now in his sixth year as Fremantle coach, but their only finals performance to date came in 2022.
King has taken regular pot shots at Longmuir over the years, especially for things like the team's slow ball movement, and it hasn't gone unnoticed.
"If I had a dollar for every time someone asked me what I've done to David King, I'd be a rich man," Longmuir said on Thursday.
"I'd probably be in the Bahamas, guts up, rather than doing this job.
"But in the end, everyone needs to understand that it's just David's opinion.
"I don't want to be disrespectful towards anyone, but I don't think David's put together a game plan, put together a list, put together a culture.
"It's just an opinion, and that's what I take it as."
Longmuir has guided Fremantle to 63 wins, 58 losses and two draws in his tenure.
But their dramatic fall from grace at the tail-end of last year - they lost their last four games to crash from the top four to 10th on the ladder - raised the pressure on Longmuir heading into this season.
When asked how he felt about the severity of criticism levelled at him despite only being a win out of the top four, Longmuir replied: "It seems a little bit over the top, but it seems to be his way with me."
Longmuir said the intense criticism directed at coaches could lead to some brilliant football brains opting out of a coaching pathway altogether.
The death threats levelled at Carlton coach Michael Voss hammered home just how much heat coaches can cop.
"I understand there's a lot of passion in our game. I love it. That passion drives our game," Longmuir said.
"But there's a line you can't cross. Clearly a line's been crossed there.
"In the end it is just a game. I know we all want our teams to win every week and finish on top of the ladder and win the flag every year, I understand that.
"If he's getting death threats, that's going way too far."
